1. Field of the Invention
The present invention relates to transducers. More particularly, the present invention relates to arrays of audio speakers, microphones, or other sensors or transducers.
2. Description of the Related Art
Audio speakers continually undergo revisions in attempts to balance aesthetic appeal, sound quality, enclosure configurations, and manufacturing cost. Recent trends have focused on providing an array of speakers to optimize cost, style, number of drivers and power considerations. Generally, the array has been formed in a line, i.e., a “linear array”. Unfortunately, the frequency response of a linear array is not nearly as omnidirectional as that of a single driver. Speaker arrays having a plurality of speaker drivers are nonetheless popular because of their ability to increase the sound pressure level (SPL) in direct proportion to the number of drivers, thereby providing SPLs comparable to that of larger single drivers while using inexpensive small drivers. Their popularity is also due in part to the styling flexibility they provide.
The most basic configuration of a line array includes a group of speaker drivers arranged in a straight line with uniform spacing between the drivers, and with the drivers operating with equal amplitude and in phase. Other configurations involve out of phase electrical coupling of the drivers but these configurations usually compromise the output power. The basic configuration generally displays omnidirectional characteristics at low frequencies but exhibits attenuation and response notches or troughs at higher frequencies and off-axis positions. This response behavior is often referred to as “lobing”. That is, as the wavelengths of the respective frequencies reproduced approach the spacing between the speaker drivers, the uniform response disappears. This occurs because the sound characteristics at any position and frequency are a function of constructive and destructive interference caused by the sound waves emanating from the individual drivers in the array. Generally, the sound waves combine constructively on axis, i.e., at a normal to a line passing through the array drivers. For off-axis positions, i.e., at angles non-orthogonal to the line passing through the array drivers, frequency-dependent destructive interference can occur.
Destructive interference is significant in its effects on the frequency response of the array, particularly for a listener who is moving or in a listening position perhaps close to the ideal position but not precisely at the optimal position. This optimal listening position has generally been referred to as the sweet spot of a speaker or a group of speakers and generally includes on-axis positions. As the angle to the listener departs from the normal (on-axis) position, the destructive interference effects become more apparent. Particularly with increasing frequencies, the effects from the destructive interference are more pronounced, resulting in smaller sweet spots or regions.
Methods in the prior art require frequency-selective filtering, weighting, and/or out-of-phase coupling of the elements, all of which compromise the broadband output power.
It is therefore desirable to provide an array of speakers having an improved frequency response over a wider range of off-axis angles and hence an increased sweet spot. It is furthermore desirable to provide such an improved frequency response while minimally compromising the output power of the array.